1. Field of the Invention
The present invention relates to an image forming apparatus and light intensity correction method.
2. Description of Related Art
Lately, an image forming apparatus which uses an LED printer head (hereinafter referred to as LPH) has been developed as an optical writing apparatus to form an electrostatic latent image on a surface of a photoreceptor drum. The LPH includes an LED chip array and lens array. In the LED chip array, LED chips including a plurality of Light Emitting Diode (LED) elements aligned according to a predetermined resolution along a main scanning direction are arranged in an array. A plurality of Graded Index (GRIN) lenses which form the electrostatic latent image on the photoreceptor drum by gathering exposed light emitted from the LED elements according to the image data are arranged in the lens array.
It is well known that light intensity unevenness occurs in an image forming apparatus which uses the LPH and the light intensity unevenness occurs due to variation in manufacturing of the LED element, variation in optical characteristic due to angle movement when fixing the GRIN lens and refractive index distribution, attached dust, etc. There is a problem that the light intensity unevenness causes density unevenness which further causes a black streak or white streak on the image.
Japanese Patent Application Laid-Open Publication No. 2005-59356 discloses an image forming apparatus provided with an LPH including a light emitting device including a first correction value storage section to store an element correction value corresponding to each LED in order to prevent an image flaw by black streak in the sub-scanning direction or density unevenness due to unevenness of light intensity of the LED or characteristic variation of the lens, and a second correction value storage section to store chip correction value corresponding to an LED chip, where correction data is generated by the element correction value read out from the first correction value storage section and the chip correction value read out from the second correction value storage section and the light intensity of light emission of the LED is corrected.
Also, Japanese Patent Application Laid-Open Publication No. 2005-28726 discloses an image forming apparatus using an LPH where an image is formed while moving the LPH in the main scanning direction and the formed image is output on a sheet in order to identify an image flaw in which a black streak or density unevenness occurs due to light intensity unevenness of the LED and to perform correction. Then, an image including a predetermined angle with respect to the paper conveying direction is read from the image output on the sheet and the density unevenness due to the LPH is detected. The correction value of the LPH is obtained based on the detected density unevenness and the exposed light intensity of the LPH is adjusted based on the obtained correction value.
However, it is known that the light intensity correction value of the LPH to reduce black streaks due to the lens array used in the LPH is different according to the type of image formed (for example, screen image which expresses grayscale representation), number of screen ruling, image density, etc. Therefore, there is a demand for a user to be able to adjust the correction value at a setting of an image density (especially grayscale) desired by the user of the image forming apparatus.